The depth of a downhole tool in the well bore is typically determined by measuring the length of a suspension cable attached to the tool that has been lowered into the well bore. This depth measurement is made by an encoder or odometer which measures the amount of cable that has been paid out from a winch into the well bore. Unfortunately, the odometer measurement does not accurately reflect the downhole location of the tools or whether the tool is actually moving within the well bore. The odometer measurement contains errors which result from stretching of the cable between the winch and the downhole tool. Cable stretch is affected by many factors including the type of cable used to suspend the tool, the tool weight, the tension in cable, cable friction, hole rugosity, deviation of the hole, temperature in the well, speed of movement and changes in speed of the cable, and variations in cable tension caused by the friction between the cable and the casing and between the tool and the casing, as well as other factors. Compensation for some of these factors can be made, but not for all of these factors. As a consequence, the measurement from the odometer is not a reliable indication of the depth of the tool within the well bore.
The traditional technique of augmenting the odometer measurement is using casing collar location information obtained from a casing collar locator carried by the downhole tool. The casing collar locator magnetically senses the position of casing collars where connected segments of pipe are threaded together to form the casing. A casing collar is an enlarged diameter end portion of one pipe segment which is internally threaded and which receives the external threats at the end of the adjoining connected pipe segment. The larger diameter of the casing collar and the possibility of a slight gap between the adjoining ends of the connected pipe segments creates a distinctive change in the magnetic permeability of the metal (typically steel) casing at each collar. The change in magnetic permeability is detected by signals from the casing collar locator when the downhole tool moves past the casing collar. The casing collar signals are communicated to the surface and coordinated with the odometer information to form a log or record of the odometer-measured depth of the casing collar locations.
Because the casing collar signals reliably identify the location of each casing collar, and because each pipe segment of the casing has a definite length, typically 30 or 40 feet, reasonably reliable indications of the depth of the downhole tool are established at intervals of 30 or 40 feet by the casing collar signals, despite inaccuracies of the odometer measurement. The locations of the casing collars are fixed relative to the surrounding earth formation as a result of cementing the casing into the borehole drilled into the earth formation.
The principal record of depth information for a well is called a “base cased hole log.” All downhole functions are performed relative to the depth reference shown on the base cased hole log. The base cased hole log includes depth information derived from the casing collar signals, and information derived from two natural gamma ray surveys, one taken in the open borehole before the casing is inserted and the other taken after the casing has been cemented in place. The rocks and natural geological structures surrounding the borehole emit different intensities of natural radiation at different depths, and a gamma ray detector of a downhole tool detects the variation in gamma ray radiation relative to the odometer-measured depth. The resolution of a natural gamma ray survey is in the neighborhood of 0.5-1.0 feet. The open borehole natural gamma ray survey is sometimes accompanied by other types of surveys which log other formation parameters. The base cased hole log is created by correlating the natural gamma ray survey intensity information, the casing collar location information, the number and length of the pipe segments which form the casing, and any other open borehole survey information. The base cased hole log thereafter serves as the master depth reference log or base tie log for the well.
The depth information of the base cased hole log may not be precisely accurate, but that information nevertheless is used as the controlling reference for all subsequent depth measurements in the well bore. Certain downhole tools which are used for specialized purposes in a well bore are capable of providing high depth resolution. However, the use of such specialized downhole tools is an unusual event and the depth information from such tools is typically not used to create the base cased hole log or in subsequent downhole functions or activities.
Subsequent downhole functions or activities at predetermined depths require the depth information from the odometer to be correlated or tied in to the depth information of the base cased hole log. The most reliable interval depth information by which to correlate presently obtained odometer depth information to the depth information shown on the base cased hole log is casing collar location information. To obtain casing collar location information to achieve such correlation, a downhole tool that is used to perform the subsequent downhole function includes a casing collar locator. As the downhole tool moves past the casing collars, the casing collar locator sends casing collar signals to the earth surface where those signals are correlated to the odometer measured depth and to the base cased hole log.
If positioning is required between casing collar locations, the odometer must be relied upon to determine those intermediate positions. However, reliance on the odometer is prone to error even over intervals as short as 30 or 40 feet, particularly when the downhole tool must be stopped and started in its movement within the well bore. For example, the inventors are aware of the situation where the odometer indicated that approximately 15 feet of the suspension cable had been withdrawn from a 12,000 foot deep well bore, thereby presumably moving the downhole tool 15 feet, but in actuality the downhole tool had not moved from its previous position due to cable stretch and other anomalies. These types of errors are significant in well bore surveys that require exact placement of the downhole tool between casing collar locations.
For example, an accurate gravity survey requires positioning the gravity measurement instrument within the downhole tool at a position which is no more than about 1 or 2 cm different from the position of the gravity measurement instrument when an earlier reading of gravity was measured supposedly at that same location. Presently, the closest resolution available is within about 3 or 4 inches. A gravity survey is typically used to determine the extent of extraction of hydrocarbon products from an underground reservoir. As the reservoir is produced, the extracted gas or oil is displaced by water. A change in density of the reservoir occurs because the heavier density water has replaced the lighter density hydrocarbon products produced from the well bore. The change in density creates a change in gravity. Measuring the change in gravity requires two time-displaced or time-lapse gravity surveys. The difference of the earlier and later gravity measurements describes the extent of hydrocarbon production. The earlier and later gravity surveys may be separated by months or years; nevertheless it is very important to conduct the surveys at the same depth positions within the well bore to obtain reliable gravity information.
Using the casing collar signals to establish such a precise depth at the location between casing collars on a repeatable and time lapse basis is virtually impossible at the present time. The different types of suspension cables and odometers used in the later depth measurements contribute to the uncertainty. Starting and stopping the survey tool at positions between casing collars greatly multiplies the uncertainty. Consequently, many downhole functions, including well bore and gravity surveys, are not performed as accurately as desired, due to the practical inability to reliably and precisely measure the depth or distance between adjacent casing collars.